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<h1> Coordinates, Points and Rectangles<br>
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The coordinate system of a Window or View has (0, 0) at the top left corner,
with <i>x</i> coordinates increasing to the right and <i>y</i> coordinates
increasing downward. The coordinates label infinitely thin horizontal and
vertical lines, and pixels exist between these lines, as illustrated in the
figure below.<br>
<br>
A <b>point</b> is represented by a 2-tuple (<i>x, y</i>). A 2-tuple can also
be used to represent as size (<i>width, height</i>) or a relative displacement
(<i>dx, dy</i>).<br>
<br>
A <b>rectangle</b> is represented by a 4-tuple (<i>left, top, right, bottom</i>).
If <i>right</i> &lt;= <i>left</i> or <i>bottom</i> &lt;= <i>top</i>, the
rectangle is an <i>empty rectangle</i> and is considered to enclose no pixels.<br>
<br>
<b>Angles</b> are measured in degrees <i>anticlockwise</i> from the <i>x</i>-axis.<br>
<br>
The <a href="Geometry.html">Geometry</a> submodule provides a collection
of functions for operating on points and rectangles.<br>
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<div align="center"><img src="coordinates.jpg" alt="" height="360" width="576">
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<small><font face="Helvetica, Arial, sans-serif">Relationship between coordinates,
pixels, points and rectangles</font></small><br>
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